如何使用PHP / mySQL查询一个表中存在的条目而不是另一个表中的条目？

Question

I want to query only the entries in table2 which contains a "course" value and "course" does not exist in table1. I initially inner joined table1 to table2 based on the "course" value. Here's what I have so far, which doesn't work:

    $query = "SELECT value1,value2,value3 FROM table2 INNER JOIN table1 USING(course)
            WHERE table2.sem = '$semester' AND NOT EXISTS (SELECT course FROM table1)
            ORDER BY course";
    $result = mysql_query($query2) or die(mysql_error());

    while ($row2 = mysql_fetch_array($result))
    {
        print_r($row);echo "<br><br>";
    }

This inner join code works:

    $query = "SELECT * FROM table1 INNER JOIN table2 USING(course) 
            WHERE table1.sem = '$semester'
            ORDER BY course";
    $result = mysql_query($query) or die(mysql_error());

Answer 1

$query = "SELECT value1,value2,value3 FROM table2 
            WHERE table2.sem = '$semester' AND course IS NOT NULL
            AND course NOT IN(SELECT course FROM table1)
            ORDER BY course";

EDIT: If you want an in depth explanation as to why to go this route instead of the left join (which will also work) check out this article:

http://explainextended.com/2009/09/15/not-in-vs-not-exists-vs-left-join-is-null-sql-server/